remove JET runtime dispatch error

[matthijscox-asml]

I am hunting JET errors in my code and I found the following error in CategoricalArrays that's easy to fix with a tiny rewrite.

This is how I found the JET error:

julia> using JET, CategoricalArrays

julia> @report_opt CategoricalVector{String, UInt8}(String[])
...
│││││││││││┌ push_level!(pool::CategoricalPool{String, UInt8, CategoricalValue{String, UInt8}}, level::String) @ CategoricalArrays C:\Localdata\Repositories\CategoricalArrays.jl\src\pool.jl:71
││││││││││││ runtime dispatch detected: CategoricalArrays.hash(level::String, %33::Union{Nothing, UInt64})::UInt64  
│││││││││││└────────────────────

JET thinks a Nothing might pass into the code, even though the code checks for a nothing. With my change JET doesn't make that mistake anymore. I understand it's a trivial change, but do hope you'll allow this.

[matthijscox-asml]

Plots.jl fails to precompile for latest Julia on x86 windows :(

Is it possible to accept this PR with that unrelated failure?

[nalimilan]

Thanks! It's not great to have to do this kind of change but it seems harmless enough.